Improve typing of generics

As discussed in [this](https://github.com/Bogdanp/dramatiq/pull/744#discussion_r2251043238) PR discussion we want to add
```diff
diff --git a/mypy.ini b/mypy.ini
index 7c2cbac..a73163a 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -3,6 +3,7 @@ pretty = True
show_column_numbers = True
show_error_codes = True
show_error_context = True
+disallow_any_generics = True

[mypy-gevent]
ignore_missing_imports = True
```
and fix the fallout of
```
Found 23 errors in 9 files (checked 89 source files)
```